When a person's larynx has been removed by surgery due to pathological changes in the throat, the trachea is sutured to an opening in the throat (tracheostoma). By the surgery the person has lost the ability to speak, and in order to restore this ability a method has been applied for several years, in a voice prosthesis of the kind referred to above is mounted in a fistula, i.e. a passage between trachea and esophagus. At speech the tracheostoma is occluded by sealing the same either by the patient placing the fingers against the tracheostoma or by the tracheostoma being closed by an in connection therewith provided stoma valve. Then, the expiration air is pressed from the lungs through the voice prosthesis into esophagus where the mucous membranes of the throat are brought into vibration and speech is produced as a consequence thereof. Several voice prosthesis are described in U.S. Pat. No. 4,911,716, U.S. Pat. No. 4,435,853, U.S. Pat. No. 4,820,304, and DD-Al-275 183. The voice prosthesis is fixed in the fistula by means of two flanges on the spool-shaped element or by means of a flange and ribbons. The spool-shaped element can be cylindrical or oval and preferably it is made of silicon rubber. All existing voice prostheses have in common that they provide a check valve function, which means that the valve mechanism normally is closed but opens when air is pressed from trachea via the valve to esophagus. The valve mechanism is maintained in the closed position by spring bias which in most cases is maintained by elasticity of the material from which the voice prosthesis is made.
When existing voice prostheses are mounted in the fistula the function is acceptable initially but they have a non-acceptably short life ranging from a week up to two years. There are two reasons for this short life, viz.                1) Growth of fungus, candida, will cover the sealing surfaces of the valve mechanism, which causes leakage at the intake of beverages and is the primary reason for exchange of the voice prosthesis.        2) Fatigue of the spring bias (the material of the voice prosthesis) so that the valve as a consequence thereof will be partly open in the normal position causing leakage through the voice prosthesis.        
A further drawback adds to these drawbacks viz. that the spring bias which shall maintain the valve mechanism closed in the normal position increases with the opening movement of the valve mechanism so that a considerably increased air resistance will be obtained as a consequence thereof at increased air flow, which means that the patient will be restrained from producing a more powerful speech or in any case cannot produce such speech without great effort, which makes the speech strenuous.
In existing voice prostheses the valve mechanism opens relatively quickly and preferably at low pressure when air with the stoma closed is pressed against the valve mechanism. Then, when the air flow is increased at speech the flow resistance will quickly be increased as mentioned above.
It is known per se to provide a valve mechanism with a permanent magnet function in order to maintain the valve mechanism in closed condition. Thus, there is disclosed in WO 93/11820 a cough valve having a valve flap which is kept in closed position against a seat by the force of a permanent magnet, such force acting between the valve flap and the seat, in order to be opened at a rising high overpressure in trachea such as at an attack of coughing. It is thus the question of a pure safety valve wherein the permanent magnet function is utilized for a completely other purpose than for overcoming the drawbacks connected with existing voice prostheses, which have been discussed above and which are not at all mentioned in WO 93/11820.
As mentioned above the growth of candida on the sealing surfaces of the voice prosthesis is the main reason for the necessity of exchanging the voice prosthesis. It is well known that the candida fungus affects silicon rubber which is the material almost exclusively used in voice prostheses, and above all at such places which are in contact with the mucous membranes in the throat.